I. Field of the Invention
This invention relates generally to an apparatus and method for controllable synthesis of noble metal monolayer electrocatalysts involving the redox displacement of an adlayer of a non-noble metal, such as Cu, by a noble-metal, such as Pt.
II. Background of the Related Art
Metals such as platinum (Pt), palladium (Pd), ruthenium (Ru), and related alloys are known to be excellent catalysts. When incorporated in electrodes of an electrochemical device such as a fuel cell, these materials function as electrocatalysts since they accelerate electrochemical reactions at electrode surfaces yet are not themselves consumed by the overall reaction. Although noble metals have been shown to be some of the best electrocatalysts, their successful implementation in commercially available energy conversion devices is hindered by their high cost in combination with other factors such as a susceptibility to carbon monoxide (CO) poisoning, poor stability under cyclic loading, and the relatively slow kinetics of the oxygen reduction reaction (ORR).
A variety of approaches has been employed in attempting to address these issues. One approach involves increasing the overall surface area available for reaction by forming particles with nanometer-scale dimensions. Loading of more expensive noble metals such as Pt has been further reduced by forming nanoparticles from alloys comprised of Pt and a low-cost component. Still further improvements have been attained by forming core-shell nanoparticles in which a core particle is coated with a thin shell of a different material which functions as the electrocatalyst. The core is usually a low cost material which is easily fabricated whereas the shell comprises a more catalytically active noble metal. An example is provided by U.S. Pat. No. 6,670,301 to Adzic, et al. which discloses a process for depositing a thin film of Pt on dispersed Ru nanoparticles supported by carbon (C) substrates. Another example is U.S. Patent Appl. Publ. No. 2006/0135359 to Adzic, et al. which discloses platinum- and platinum-alloy coated palladium and palladium alloy nanoparticles. Each of the aforementioned is incorporated by reference in its entirety as if fully set forth in this specification. Although these approaches have produced catalysts with a higher catalytic activity and reduced noble metal loading, realization of these enhancements on a commercial scale requires the development of large-scale and cost-effective manufacturing capabilities.
Practical synthesis of electrocatalyst particles with peak activity levels requires the development of commercially viable processes which are still capable of providing atomic-level control over the formation of ultrathin surface layers. Such a process must allow formation of uniform and conformal atomic-layer coatings of the desired material on a large number of three-dimensional particles having sizes as small as a few nanometers. One method of depositing a monolayer of Pt on particles of different metals involves the initial deposition of an atomic monolayer of a metal such as copper (Cu) by underpotential deposition (UPD). This is followed by galvanic displacement of the underlying Cu atoms by a more noble metal such as Pt as disclosed, for example, in U.S. Patent Application Publ. No. 2007/0264189 to Adzic, et al. Another method involves hydrogen adsorption-induced deposition of a monolayer of metal atoms on noble metal particles as described, for example, in U.S. Pat. No. 7,507,495 to Wang, et al. Yet another mechanism involves an apparatus and method for the synthesis and treatment of metal monolayer electrocatalyst particles in batch or continuous fashion, as described in PCT Patent Publication No. WO/2011/119818 to Adzic et al. Each of the aforementioned is incorporated by reference in its entirety as if fully set forth in this specification.
Although these processes have been successful for small-scale experiments performed in the laboratory, their commercial realization will require the development of systems and methods capable of processing a large number of electrocatalyst particles to within very tight tolerances. There, therefore, is a continuing need in the art for the development of commercially viable systems and methods for synthesizing electrocatalyst particles.